In a typical wireless, cellular or radio communications network, wireless devices, also known as mobile stations, terminals, and/or User Equipment, UEs, communicate via a Radio-Access Network, RAN, with one or more core networks. The RAN covers a geographical area which is divided into cells, with each cell being served by a base station, e.g. a radio base station, RBS, or network node, which in some networks may also be called, for example, a “NodeB”, “eNodeB” or “eNB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. One radio base station may serve one or more cells.
A Universal Mobile Telecommunications System, UMTS, is a third generation mobile communication system, which evolved from the second generation, 2G, Global System for Mobile Communications, GSM. The UMTS terrestrial radio-access network, UTRAN, is essentially a RAN using wideband code-division multiple access, WCDMA, and/or High-Speed Packet Access, HSPA, to communicate with user equipment. In a forum known as the Third Generation Partnership Project, 3GPP, telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN, as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller, RNC, or a base station controller, BSC, which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System, EPS, have been completed within the 3rd Generation Partnership Project, 3GPP, and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio-Access Network, E-UTRAN, also known as the Long-Term Evolution, LTE, radio access, and the Evolved Packet Core, EPC, also known as System Architecture Evolution, SAE, core network. E-UTRAN/LTE is a variant of a 3GPP radio-access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base station nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio-Access Network, RAN, of an EPS has an essentially flat architecture comprising radio base station nodes without reporting to RNCs.
The wireless communications network described above is most commonly arranged to operate within a licensed frequency spectrum, i.e. regulated and dedicate frequency bands in which a centralized network controls the wireless or radio communication according to a certain predetermined standard. However, recent developments has opened up to expand these wireless communications networks to also operate in parts of the so-called unlicensed spectrum, i.e. unlicensed frequency bands which are shared, decentralized and not licensed to a particular type of scheduled wireless or radio communication. Examples of wireless technologies that today utilize the unlicensed spectrum may include Ultra Wideband, spread spectrum, software-defined radio, cognitive radio, and mesh networks. In the unlicensed spectrum, wireless devices of different wireless technologies compete with each other about having access to and transmitting on the transmission resources within the spectrum. Hence, the channel sharing of these transmission resources may be referred to as contention-based. To achieve a fair channel sharing of these contention-based transmission resources within the unlicensed spectrum, principles based on Carrier Sense Multiple Access/Listen-Before-Talk, CSMA/LBT, and/or Discontinuous Transmission, DTX, may be used.
The CSMA protocol is a probabilistic MAC protocol, wherein access to the channel is achieved by first sensing the channel, and then transmitting if the channel was found to be free. The principle of CSMA may also be referred to as Listen-Before-Talk, LBT, since every network node which wants to access the channel by means of transmitting data must first listen if the channel is available or not. This principle used today in WiFi networks for achieving a fair sharing of the channel. The sensing of the channel is realized by measuring the energy of the channel over a certain period of time or, in other words, by listening to the channel. If the measured energy is found to be below a predetermined threshold, then the channel is considered to be free. In this case, there is no other ongoing data transmission, which means that the channel is not used by any other nearby node. Thus, the network node that listened to the channel is able to occupy it and use it for its data transmission.
In the opposite case, where the measured energy is above the predetermined threshold then the channel is considered to be busy and the network node is not permitted to occupy the channel. In this case, the network node will instead wait, or defer, until the channel becomes available again. Typically, a random back-off counter is employed by the network node, and the network node will measure the channel until the counter expires. If the channel is found to be free during this time period, then the network node may access it. The random back-off concept is a contention mechanism which allows multiple users to measure the channel for different time periods and avoid systematic collisions. For example, it has been proven that if two network nodes employing WiFi in a WiFi network which cover the same area, then each network node will have access to the channel half, or 50%, of the time, given a heavily loaded network, i.e. a large amount of WiFi devices attempting to gain access to the WiFi network.
FIG. 1 illustrates an example of the CSMA or LBT principle in a WiFi network. Upon data arrival, a WiFi Access Point, AP, senses the channel for a period equal to Distributed Inter Frame Space, DIFS. A typical duration of the DIFS is about 34 μsec. If the channel is found to be free during this sensing or DIFS period, or in other words, the measured energy is less than a threshold, then the WiFi AP starts to transmit data to the user. An example of a typical value for the threshold is about −82 dBm. Upon successful reception of the data, the user transmits an acknowledgement message, ACK, after a period equal to Short Inter Frame Space, SIFS. A typical duration of the SIFS is about 16 μsec. After the ACK reception, or after a random back-off time, a new sensing period, or DIFS period, occur. In this example, the channel is found to be busy and the WiFi AP defers its new data transmission until the channel is found to be free again. When the channel is found to be free again, a new sensing period or DIFS period occur. If the channel is found to be free during this sensing or DIFS period, the WiFi AP starts to transmit the new data. Optionally, a random back-off timer might be added in the last DIFS to avoid simultaneous transmissions of two different deferred APs. The random back-off time is measured in units of slots with a typical value of 20 μsec.
In employing LTE in unlicensed bands, one concept builds upon Carrier Aggregation, CA, where a licensed based LTE carrier is aggregated with an unlicensed LTE carrier. In this case, the important control signaling is transmitted through the licensed LTE Component Carrier, LTE CC, and the unlicensed CC, or Secondary CC, SCC, may be used to boost the data rate. This concept is commonly referred to as License-Assisted LTE, LA-LTE, or LTE-Unlicensed, LTE-U. LA-LTE must be able to co-exist and share the channel in a fair manner with other systems deployed also on the same unlicensed frequency bands, such as, a WiFi network. LBT is one option that may be employed by an LA-LTE network in order to provide a fair sharing of the channel between different networks. Another option for the coexistence of different networks when deploying LTE in unlicensed frequency bands is DTX, as described below.
FIG. 2 shows an example of co-existence of a LA-LTE network and a WiFi network. The LA-LTE transmissions follow an ON-OFF pattern. During an ON period, the neighbouring WiFi AP defers its data transmission since it senses that the channel occupied. During an OFF period, the WiFi AP senses that the channel is free and it will transmit data. With DTX, the network node employing LA-LTE is transmitting during specific periods of time, otherwise it will remain silent. This means that the activity of the network node follows an ON-OFF pattern, where during ON periods the network node is allowed to transmit data and/or control information, and during OFF periods, the network node must remain silent. Nevertheless, in some cases, the network node may still transmit mandatory physical layer signals during the OFF periods to maintain connectivity with the user and support user measurements. An example of a DTX scheme used in LTE is the Enhanced Inter-Cell Interference Coordination, eICIC, or Almost-Blank-Subframes, ABS. Another example is the Cell-specific Reference Signal ON-OFF, CRS ON-OFF, wherein physical layer signals are not transmitted during OFF periods. The ON-OFF patterns may be configured statically, semi-statically or completely dynamic based on the available information. The ON-OFF pattern is characterized by its duration in time or periodicity and the ON ratio which is defined as the ON time divided by the total ON-OFF period. Alternatively, the ON ratio may be defined as the number of ON subframes divided by the total number of subframes. Hence, an ON ratio of 50% means that the network node is in an ON state 50% of the total time. The OFF periods, here, defer the network node from occupying the channel and thus leave the channel free for the possibility of other neighbouring nodes to access it.
FIG. 3 shows an example of LBT implemented in a network node implementing LA-LTE network. Here, a listening LBT slot is located at the end of each LTE subframe. If the channel is found to be busy during the listening LBT slot, then the network node defers from transmitting any signal, such as, e.g. data, control, physical layer signals, until the next LBT slot where the network node will re-evaluate the channel activity. For simplification, we here assume that LBT is performed periodically with a period equal to one TTI. The duration of the listening LBT slot may, for example, be in the order of tens to hundreds of μsec. Optionally, a random duration of the listening LBT slot may be employed in order to avoid systematic collisions. The listening LBT slot duration may also correspond to a fraction or up to a few LTE OFDM symbols, where each LTE OFDM symbol has duration of about 71 μsec. Hence, in this example, each LTE subframe is divided in time into two parts, where the first part carries data and the second part wherein listening takes place. The listening occurs at the end of subframe K, i.e. LBT slot, and determines whether or not data will be transmitted in the next subframe K+1.
It should however be noted that these principles, i.e. CSMA/LBT and DTX, when applied to the physical transmission layer, i.e. the Media Access Control Physical layer, MAC-PHY layer, will impose rather harsh restrictions on when in time a network node employing LA-LTE is allowed to transmit on the contention-based transmission resources. For example, when deploying prior art DTX schemes for LTE in unlicensed frequency bands, the network node is deferred from transmitting any data or control information during the OFF periods. This means that no physical layer signaling is allowed to be transmitted. This includes signaling of CRS, PSS/SSS, CSI-RS, etc. CRS ON-OFF may, for example, be employed for implementing such a DTX scheme.
One drawback of this solution is that the network node is muted for some arbitrary time which is independent of the current data traffic of any neighbouring network nodes or access point. Hence, this will have a negative impact on its performance. Furthermore, deferring transmissions of physical signals during the OFF periods might deteriorate the performance of the network. Physical signals from connected users are used in many vital operations, such as, e.g. to identify cells, to perform measurements, to execute handovers, to perform cell changes, etc. If physical signals are not transmitted properly, then many of the above operations might fail. This may lead to out-of-synch states in users and to a reduced radio link performance. This may also be worsened in case OFF periods are long or happen too often.